The present invention relates to a method and an apparatus for desulfurizing a nitrogen oxide adsorber of an exhaust purification installation for a combustion system by way of desulfurizing phases that are implemented intermittently.
The process of employing nitrogen oxide adsorbers in waste gas purification devices for predominantly lean burning combustion systems, such as so-called low-performance vehicle engines, is known in the art. Most frequently, these adsorbers are in the form of so-called nitrogen oxide adsorber catalysts.
In the lean operation phases of a combustion system, during which the concentration of nitrogen oxide in the exhaust is increased most of the time, the nitrogen oxides are adsorbed and temporarily stored in the nitrogen oxide adsorber, predominantly in a nitrate form. When the nitrogen oxide adsorber reaches its highest utilization factor, it is switched from adsorption operation to desorption operation. The stored nitrogen oxides are once again desorbed and can be converted, e.g. in a reduction reaction, to nitrogen. In support of this process, among other things, the operation of the combustion system is converted from lean to rich. As is customary, lean or rich operation is defined as operation at an air/fuel ratio xcex, also referred to as the air ratio, that is above or below the stoichiometric value of the air/fuel mixture that is being burned in the combustion system.
A known difficulty with such systems is that, due to the presence of sulfur which is contained in most fuels and motor oils, sulfur dioxide is present in the exhaust gas. Sulfur accumulation can lead to sulfur poisoning of the nitrogen oxide adsorber, reducing the capacity of the adsorber over time to store nitrogen oxide. Thus, it is known in the art that the nitrogen oxide adsorber is to be subjected to a desulfurization procedure whenever the capacity of the adsorber to store nitrogen oxide noticeably begins to decline in order to free the adsorber from the deposited sulfate. In the present context, the term nitrogen oxide adsorber shall refer to adsorbers that are comprised of a single adsorber body and, to adsorbers in which the nitrogen oxide adsorber body is preceded by a so-called SOx trap. The SOx trap temporarily stores the sulfur oxides thereby keeping them away from the nitrogen oxide adsorber body, and similarly undergoes intermittent desulfurization.
For the desulfurization of the nitrogen oxide adsorber, the method of converting the exhaust gas composition from lean to rich is known in the art, for example, by (1) a corresponding operation conversion of the combustion system and by thereby increasing the temperature in the exhaust gas pipe, :(2) an operation of the combustion system that will result in higher exhaust gas temperatures; and/or (3) an electrical heating device. This type of desulfurization method is described in EP 0 636 770 A1 (U.S. Pat. No. 5,483,795). Other, similar desulfurization methods that contain, in addition, a secondary air intake or split operation of a multi-cylinder combustion engine, which represents the source of combustion, into one part rich combustion cylinders and the other part lean combustion cylinders, are described in DE 195 22 165 A1; DE 197 47 222.2 (U.S. Pat. No. 6,161,377); and DE 198 02 631.5 (U.S. Pat. No. 6,119,450).
During the desulfurization of the nitrogen oxide adsorber, sulfur compounds, with sulfur dioxide and hydrogen sulfide occurring predominantly, get into the exhaust gas that is exiting the nitrogen oxide adsorber. Hydrogen sulfide, as is generally known, has a pungent odor and represents odor pollution even at relatively low concentrations.
The present invention relates to the technical problem of providing a method and an apparatus that will allow for the effective desulfurization of the nitrogen oxide adsorber, while ensuring that steps are taken for minimizing the hydrogen sulfide emissions.
With the method according to the present invention and the apparatus of the present invention, the concentration of carbon monoxide (CO) and/or the lambda value of the exhaust gas stream exiting the nitrogen oxide adsorber are recorded during a respective desulfurization phase.
Studies have shown that, after the start of a desulfurization phase involving the conversion from a lean to a rich exhaust gas composition, a release of sulfur dioxide (SO2) occurs initially. Then, over the course of this release of sulfur dioxide, the carbon monoxide concentration in the exhaust gas begins to rise to a localized maximum from which point it falls again. On the other hand, the lambda value of the exhaust gas drops at first only as far as a temporary plateau value in the range of the stoichiometric value one where it remains for some time, before dropping at a visibly steep slope to a corresponding target value. Only after the appearance of this localized maximum in the CO concentration curve or of the drop of the slope of the lambda value from the plateau value to the target value, begins the noticeable formation of hydrogen sulfide (H2S). This knowledge is used in the present instance to detect during the desulfurization phase, by recording the CO content, the appearance of the localized maximum or, by recording the lambda value, the appearance of the downward gradual slope and to use this data as a criterion for the termination of the desorption phase.
This allows prevention of the premature termination of the desulfurization phase or, in terms of hydrogen sulfide emissions, the belated termination of the respective desulfurization phase. If the phase is terminated prematurely, regeneration would remain incomplete, even if, as studies have shown, multiple, consecutive repetitions of such abbreviated desulfurization phases are implemented. If the desulfurization phase last too long, perceptible quantities of hydrogen sulfide would form that would undesirably be emitted along with the exhaust gas or would have to be rendered harmless at additional effort and expense.
In contrast, by terminating the desulfurization phase at the time the localized maximum of the CO concentration curve is reached or at the time the downward slope of the lambda value curve occurs, or at a defined time slightly after that time, it is possible to achieve an essentially complete desulfurization and therefore regeneration of the nitrogen oxide adsorber. In addition, the formation of perceptible quantities of hydrogen sulfide can be prevented without additional measures. Depending on the specific application, only one desulfurization phase or several consecutive desulfurization phases of this type can be envisioned to ensure complete desulfurization of the nitrogen oxide adsorber. These phases are interrupted, respectively, by a brief conversions to a lean exhaust gas composition.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings.